1. Field of the Invention:
This invention relates to sampled-value code processing devices and, more particularly, to a sampled-value code processing device for transmitting only sampled values determined according to a prescribed rule.
2. Description of the Related Art:
Recently, as a technology of defining images to ever higher level advances, for example, a television image which comes in daily life is transforming from the NTSC type to the high-definition television type. When such a high-definition image is recorded on a recording medium such as tape or disc, or transmitted to a remote place through a satellite or optical fibers, on consideration of the image quality deterioration factors such as S/N and jitter, the digital transmission is rather advantageous than the analog transmission.
Yet, even when performing the digital transmission, if, simply, the only measure taken is that the analog image signal is converted into a digital form to be transmitted, there are difficulties in the point of the capabilities of a transmission path (the capacity of a recording medium in the recording time, the capacitance of a communication medium and the cost). For example, if an attempt is made to secure not less than 30 MHz for the frequency band of a video signal, the sampling theorem teaches that the sampling must be done at a rate of at least 60 MHz. Suppose the A/D conversion is carried out with 74.25 MHz and 8 bits, then the transmission rate becomes 74.25(MHz).times.8(bit) =594(Mbit/s).
In the conventional digital image transmission system, a method of compressing such huge information quantity without damaging the image quality has been considered. Its typical example is a sub-sampling method in which sampled values of picture elements are thinned out by utilizing the correlation of the picture elements and according to a given rule. Referring to FIG. 1, lateral lines N-2, N-1 and N are consecutive horizontal scanning lines, with a mark "o" and a mark "x" representing the picture elements picked up at a sampling frequency fs. It should be noted that the picture elements of the mark "x", when in sub-sampling, are left out by thinning-out and are not transmitted. On the receiver side, they are restored from the picture elements of the mark "o" by interpolation, etc. The method of thinning out picture elements with one picture element shifted line by line, as shown in FIG. 1, is called the "line-offset sub-sampling" method. By using this, the information quantity is halved.
FIG. 2 is a block diagram illustrating the principle of a circuit for sub-sampling the video signal to be transmitted. The circuit comprises an analog low-pass filter 10 for band limitation before sampling, an A/D converter 12, a digital spatial filter 14 for band limitation before sub-sampling, and a sub-sampling circuit 16. The spatial filter 14 is a very important circuit which determines the image quality after the sub-sampling. Concerning the filter structure, the number of taps, etc., many systems as adapted to the sub-sampling processes have been proposed. FIGS. 3(a), 3(b) and 3(c) represent steps of the sub-sampling process performed by the circuit of the construction shown in FIG. 2. FIG. 3(a) shows picture elements at the output of the A/D converter 12; FIG. 3(b) shows picture elements at the output of the spatial filter 14, and FIG. 3(c) shows picture elements at the output of the sampling circuit 16. In short, the original picture elements of FIG. 3(a) are all subjected to filtering by the spatial filter 14, becoming the filtered picture elements of FIG. 3(b). Then, in the sampling circuit 16, the filtered picture elements are thinned out to be left out every other picture element. Thus, what is shown in FIG. 3(c) is obtained.
The method of producing the filtered picture elements by means of the spatial filter 14 is briefly explained. With respect to the picture element, for example, x.sub.6 shown in FIG. 4, a filtered picture element x.sub.6 is produced by computation based on the following formula: EQU x=(1/A)
{k.sub.5 x.sub.6 + k.sub.4 (x.sub.4 +x.sub.8) + k.sub.3 (x.sub.5 +x.sub.7)
+ k.sub.2 (x.sub.2 +x.sub.10) - k.sub.1 (x.sub.1 +x.sub.9 +x.sub.3 +x.sub.11)}. . . (1)
where x.sub.1 through x.sub.11 are original picture element data, k.sub.1 through k.sub.5 are positive coefficients, and A is a positive constant. By recycling this computation for every one picture element at the sampling frequency fs, each filtered picture element of FIG. 3(b) can be obtained.
FIG. 5 is a block diagram of the ordinary spatial filter 14 comprising an input terminal 20, 1-line or 1.sub.h (horizontal scanning period) delay circuits 22 and 24, 1-picture element (l/fs) delay circuits 26, 27, 28, 29, 30, 32, 33, 34, 35, 36, 37, 40, 41, 42, 43, 44 which are disposed before the sub-sampling process, adders 46, 48, 50, 52, 54, 56, 58, 60 and 62, a subtractor 64, multipliers 66, 68, 70, 72 and 74 which have coefficients k.sub.1 to k.sub.5, respectively, a divider having a divisor A, and an output terminal 78. As the multipliers 66, 68, 70, 72 and 74 use is generally made of such a ROM that the multiplication relationship of the coefficients k.sub.1 -k.sub.5 is established between the memory addresses and the memorized values. The outputs of the adders 56, 48, 50 and 52 are respectively (x.sub.1 +x.sub.9 +x.sub.3 +x.sub.11). (x.sub.2 +x.sub.10), (x.sub.5 +x.sub.7) and (x.sub.4 +x.sub.8). This circuit realizes the computation of the above-defined equation (1) with fidelity. The filtered picture element value x.sub.6 with respect to the original picture element x.sub.6 is output from the output terminal 78.
However, in such a conventional circuit as described above, the circuit scale gets very large. Further, all the circuit elements of FIG. 5 have to operate at the sampling frequency fs. These constitute a drawback that when the sampling frequency fs is 74.25 MHz, TTL (transistor-transistor logic) or CMOS (complementary metal-oxide semiconductor) elements do not work. Hence, use must be made of high-speed operable ECL (Emitter Coupled Logic) elements. If the ECL element is used, enormous electrical energy is consumed, and a great scale of circuit results.